Patients with ventricular fibrillation (VF) face immediate death (within minutes), if a defibrillation shock is not delivered soon after VF is detected.
One conventional defibrillation technique is to administer a defibrillation shock externally, using "paddles" or adhesive electrodes. Another conventional defibrillation technique is to administer a defibrillation shock internally, using intracardial electrodes carried by catheters, also called "internal cardioversion."
Because blood pressure drops immediately after VF occurs, the patient begins to lose consciousness within seconds after VF starts. Thus automatic implantable defibrillator shocks are often delivered to an unconscious or only partially conscious patient. Even so, shocks are painful and very disagreeable for these patients. Most patients tolerate these devices, because the alternative is nearly certain death. Although it is not well publicized, some patients do chose to have their devices inactivated, i.e. they chose to die rather than experience repeated shocks to save their lives.
Atrial fibrillation (AF) in humans, once induced for diagnostic reasons or occurring spontaneously, is often difficult to terminate. AF can also be converted to normal sinus rhythm using defibrillation shocks. Research suggests that defibrillation shocks administered to the atria can be painful to the patient if the delivered energy per pulse is greater than 1-1.5 Joules. The research also indicates that (at least with existing lead/pulse designs) defibrillation efforts are often ineffective unless energy pulses higher than 1 Joule are required. Thus, convention defibrillation of AF is often accompanied by moderate-to-intense pain for the patient.
In contrast to VF, the onset of AF, except in rare cases, does not cause immediate death. Instead, atrial fibrillation causes a long-term increased risk of death or stroke. Therefore, treatment for atrial fibrillation is usually not urgent. Restoring sinus rhythm or other organized rhythm within minutes or even hours is acceptable treatment for the vast majority of those patients.
For these reasons, delivering painful, disagreeable shocks automatically to these patients is harder to justify in the atrial defibrillation patient. Furthermore, in this patient population, the shock strengths that are required to defibrillate a patient are typically not well tolerated by an awake patient. Even partially sedated patients perceive the shocks as very painful, and typically will not willingly undergo multiple defibrillation shocks even in the controlled setting of the electrophysiological laboratory. When asked, patients candidly report that they can tolerate shocks at the rate of one per week to even one per year, with the tolerance level varying greatly among patients. To be widely tolerated, shock intensities need to be decreased substantially.
With convention defibrillation techniques, there is also a risk of induced ventricular fibrillation, possible unintended dislodgement of thrombus in one of the atria, and, in the case of internal cardioversion, the need to place one or more additional catheters within the heart.
An alternative conventional technique injects a bolus of an antiarrhythmic drug to attempt to convert the fibrillation to normal sinus rhythm. The disadvantages of this approach are low success rates and possible side-effects or reactions associated with the drug. The antiarrhythmic drug may also alter the heart's electrical functioning for a period of time, so that an electrophysiological study of the arrhythmic episode must be either delayed or canceled.
All these considerations surrounding conventional defibrillation techniques must be taken into account when the physician would like to induce and then terminate AF numerous times in an effort to gather information that would guide subsequent therapy.
Another technique being developed to defibrillate a patient in AF involves administering one or more defibrillation pulses (shocks) internally via one or more lead/electrode systems connected to an implanted sensor/pulse generator device. This system is generically referred to as an atrial implantable defibrillator (which will be called in shorthand "AID").
The AID concept has a number of potential shortcomings. They include:
(i) Pain is still likely to be encountered whenever the AID discharges to apply a defibrillation shock greater than 1 Joule in intensity;
(ii) Battery life is a key consideration in determining the clinical viability of any implantable device. During the evolution of implantable ventricular defibrillators, it was demonstrated that many ventricular tachycardia (VT) episodes could be terminated by pacing rather than by the administration of a defibrillation shock. When it is possible, not only does this spare the patient the pain associated with the shock, it also conserves battery life since (in the ventricular situation at least) a pacing train requires significantly less energy than a defibrillation shock. These facts have made possible implantable ventricular defibrillators that offer both longer battery life and reduced size.
(iii) There is always a possibility that a shock administered in an attempt to defibrillate the atria may inadvertently cause ventricular fibrillation. Although the likelihood of this event may be minimized by controlling the timing of the shock relative to the cardiac cycle, it may not be possible to entirely eliminate the risk. AID's currently do not have backup ventricular defibrillation capability.